Ion implantation is a process used to dope impurity ions into a substrate to obtain desired device characteristics. Typically, an ion beam is extracted from an ion source chamber toward a substrate. The depth of implantation of the ions into the substrate is based on the ion implant energy and the mass of the ions generated. One or more ion species may be implanted at different energy and dose levels to obtain desired device structures. In addition, the beam dose (the amount of ions implanted in the substrate) and the beam current (the uniformity of the ion beam) can be manipulated to provide a desired doping profile in the substrate. The current density of the beam and the time that a substrate is exposed to the ion beam determines the dose.
An ion implanter may generate an ion beam having a roughly circular or elliptical cross sectional shape that is smaller than the surface of the substrate to be treated. The substrate to be treated such as, for example, a semiconductor substrate may have a disk shape. In order to implant ions into substantially all of the substrate, the ion beam is scanned back and forth across the substrate at scanning frequencies up to 1 kHz. In particular, an angle corrector may be included in the implanter that is configured to accept the scanned spot beam with diverging trajectories and corrects the trajectories to provide parallel trajectories directed toward the substrate. The substrate may be mechanically driven in a direction orthogonal to the direction of the scan plane in order to treat substantially all of the surface of the substrate. For example, if the scan plane is a horizontal plane the substrate may be mechanically driven in a vertical direction so the entirety of a front surface of the substrate is treated with the ion beam as it is scanned back and forth horizontally across the surface.
The scan rate of the ion beam may be controlled to obtain a desired beam dose and consequently a particular doping profile for the substrate. The beam dose may be controlled using real time dose control techniques, such as, for example, orthogonal scan compensation (OSC), which is used to measure the beam current at various points during implantation. The time that a particular portion of the substrate is exposed to the ion spot beam can then be adjusted based on the measured beam current to ensure that the desired beam dose is consistent over the entire substrate. Thus, measuring the beam current during substrate processing requires that the ion beam be scanned a sufficient distance off of the substrate and over a fixed beam current sensor, such as, for example, a Faraday cup to provide real time dose information. However, because beam current sensors may be in a fixed position and the front surface of the disk shaped wafer typically has a substantially circular perimeter, the ion beam spends a certain amount of time off the wafer in order for the beam to be incident on the sensor. This is sometimes referred to as “oversweep” where the beam sweeps outside or off the surface of the wafer. This may result in lower beam utilization. In particular, the ratio of the substrate area to the ion implant area (i.e. the area the ion beam is swept over) is referred to as “ion beam utilization.” Thus, the more the ion beam is not incident on the substrate, the lower the ion beam utilization. The lower the ion beam utilization, the lower the device manufacturing throughput. As such, extra time and/or materials are required for ion implant processes that have lower ion beam utilization than would be for ion implant processes having higher ion beam utilization. Thus, there is a need to increase ion beam utilization. It is with respect to these and other considerations that the present improvements have been needed.